Oak Moths Are Making A Mess Of Things

I’ve been getting a lot of calls from people about “those obnoxious moths killing my oak tree.” One person was near-apoplectic with the thought of all the oaks in Santa Barbara dying. It does look pretty bleak in some parts of town. Case in point, the southbound 101 going through Montecito has a butt-load of totally brown trees – not a green leaf anywhere to be found. Virginia Hayes wrote a good piece in the Independent in June, and the Santa Barbara News-Supressed did a brief feature last week, but I think we can expand the discussion a bit.

Here’s what I have to say. This is no time to panic!!!!!! No, it’s time to grab the bull by the horns and sit idly by doing nothing. Really. As much as folks are looking for “the fix” there really ain’t one. Well, actually there is one, but it’s not worth the trouble and takes both luck and skill to pull off the cure. Cryptic enough for ya?

If the trees could speak (and were fans of Monty Python’s Holy Grail) we’d hear them shouting “But I’m not dead yet!” That’s mostly true, but a few trees that were already on their last legs just might just be pushed over the edge. Being one to see the glass as half full, my imagination sees those oaks cut into manageable pieces and glowing under a few pounds of tri-tip, spending their last moments in the service of a fine meal. Oak trees have been known for their self-sacrificing nature.

California oak moths (Namus scientificensis) have been around and evolved along with many species of California’s oak trees for a few millennia. A quick “birds and the bees” lesson: Mommy and daddy moth go out for a couple of glasses of Pinot Grigio, candles are lit, doors are closed, the girls throw mom a baby shower, and the next thing you know, a hairless little caterpillar emerges. While mom is rubbing copious amounts of vitamin-E cream into her stretch marks, the little dickens is starting to munch on the leaves of the host oak tree. No Gerber’s formula for these kids – it’s oak leaves that fill their little bellies.

Unfortunately, this doesn’t happen in isolation and in some cases thousands of babies from their play group join in the feeding frenzy on the same tree. Next thing ya know, the tree is partially or completely denuded. Add in the nuisance of caterpillar poop, called frass (keep that in mind next time you play Scrabble) collecting on nearby surfaces and I can see why folks are calling me in a panic.
Good news!

There is a biological fix in the form of a soil-dwelling bacteria (Bacillis thuringiensis) that is used to control a host of garden caterpillar pests. The most effective form these days is called Dipel and it comes in a spray or dust, available at most garden shops. Follow the directions and it’s quite safe.

Bad news – how the hell do you, the typical homeowner, effectively apply it to a mature oak tree? Hang glider? Trampoline?

More bad news – It has no effect on the mature moth. They don’t eat – too busy breeding. Now THAT’S the sign of a dedicated parent. I can see the bumper sticker on mom’s mini-van “Proud, Starving, Libido-driven Parent of an Oak-defoliating Honor Student.” So, even if you could figure out a way to spray or dust your oak, you’d have to build a tree house, sleep in the branches, arise at dawn, monitor the hatching of the pupae, and then unleash your attack.

So get over it, because (a) most healthy oaks can tolerate two or three defoliations in one growing season, (b) the moths are thought to peak in 7 to 10 year cycles (so next year shouldn’t be as bad) and (c) the tree will most likely look fine when it starts its growing cycle again after the next rains.



If your gardener says he has a spray to fix the problem, be suspicious; very suspicious. Indiscriminate spraying of toxic pesticides is a multi-edged sword that will come back and bite you, the beneficial helpers, your garden, wildlife in general, and your family.

There are beneficial insects (lady bugs, lacewings and others) and birds that are probably helping out somewhat by feeding on the caterpillars, but they’re not around in quantities that will substantially eradicate the problem.




One person told me that they go out every evening and blast the tree with a hard jet of water from their hose. They have no idea if it’s doing any good (and they’re using another precious resource as their weapon) but they sure feel great about annoying the hell out of the moths. I hope they’re ready for a visit from People for the Ethical Treatment of California Oak Moths.